Ping-Pong Avalanches

Scientists are pretty good at predicting the paths of thunderstorms, hurricanes, and even lava flow from erupting volcanoes. But the path of a deadly snow avalanche is a lot harder to predict. In this Science Update, you'll hear how ping-pong balls may help change that.

Transcript

How ping-pong balls could save a village. I'm Bob Hirshon and this is Science Update.

In mountainous areas, a snow avalanche can bury an entire town without warning. The only way to avoid such a disaster is to predict where an avalanche might go well ahead of time.

But that's easier said than done, especially for powder snow avalanches, in which much of the snow becomes airborne. To study the physics of these events, a research team in Japan dropped hundreds of thousands of ping-pong balls down an Olympic ski jump. Team member Jim McElwaine of Cambridge University explains why.

McElwaine: The ping-pong balls are quite light. So they interact with the air quite strongly. So when we dropped them down the ski slope, they started off running along the ground in contact, just supporting their own weight. But because they're quite light, they started to get picked up by the air and became suspended, in a similar way to a powder snow avalanche.

The research was led by Koichi Nishimura of Hokkaido University, and reported in New Scientist magazine. Using the ping-pong balls, the researchers figured out suprisingly simple models for the momentum and volume of the snow flow.

McElwaine and his colleagues are now testing these models on real snow in Switzerland. If they hold up, they could be used to help mountain dwellers either avoid or block future paths of destruction. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

How are ping-pong balls like snow, aside from being white? Well, as it turns out, it's not so much that a ping-pong ball is like a single snowflake, but rather that a whole bunch of ping-pong balls sliding down a slope behave like a big pile of snow.

The main reason is that ping-pong balls are light. So as they slide down an incline, they quickly get swept into the air and create a big cloud. That's very much what happens in a powder snow avalanche, the worst kind of avalanche as far as danger to humans goes (the snow can reach speeds of well over 100 mph). If you did the same experiment with bowling balls, the avalanche would be quite different (although you wouldn't want to be at the bottom of it): the bowling balls would roll pretty close to the surface of the slope.

Do the ping-pong balls model a snow avalanche exactly? No. But there are a couple of advantages to using them. First of all, ping-pong balls are a lot easier to work with: you know exactly how many you started with, how much they weigh, and so on. And because the ping-pong balls are so large compared to snowflakes, they're easier to control and measure than real snow. A snow avalanche is an incredibly complex process, mathematically speaking: the snow barrels down the slope in three dimensions, bunches up and breaks apart, and changes speed in very subtle ways. In order for scientists to understand complex processes like this, it's often useful to scale it down to a simpler model. The assumption is that if your theory doesn't work for the simpler model, it certainly won't work in real life.

Now McElwaine and his Japanese colleagues are testing their new theories on very small snow models, as well as to observations of real-life avalanches. "One of the ways you can have confidence in scientific models is if you can apply them on different scales and different situations and they still work," McElwaine explains. "That makes you really believe that you've got the physics right."

And getting the physics right could save lives. Many mountain villages live constantly under the threat of unpredictable powder snow avalanches. If this model could prove an accurate predictor of an avalanche's potential path, cities could take precautions, like building barriers that could stop or re-route the snow before it buries the town.

Now try and answer these questions:

Why are powder snow avalanches hard to predict?

Why are ping-pong balls useful as a model for these avalanches?

What are some important ways in which a ping-pong ball avalanche differs from a snow avalanche? How might this affect the math and physics behind it?

Can you think of other examples of simple models that are used to make predictions of something more complicated?